Piezoelectric materials are materials that change their shape when they are exposed to an electric field. This phenomenon is caused by the repulsive or attractive interaction of the field with the electric dipoles in the material. The nature of the shape change is dependent on the direction of the electric field with respect to the "poling" direction of the piezoelectric material. The poling direction is the direction of the dipoles in the material, which is assigned as one of the three cartesian coordinate axes.
If the electric field is applied along the poling direction, the material expands or contracts along this axis (depending on the direction of the field relative to the direction of the poling). This is known as the primary or direct piezoelectric effect. At the same time the material is expanding or contracting in the poling direction, it is contracting or expanding along the axes that are orthogonal to the poling direction. These motions are known as the secondary or indirect piezoelectric effect. The amount of motion due to the primary effect is typically greater than the amount of motion due to the secondary effect. The amount of motion due to the secondary effect in the orthogonal axes is typically isotropic.
Another property of piezoelectric materials is that, converse to the application of an electric field to cause mechanical deformation, when mechanical force is applied to the material, it produces an electrical signal. The magnitude of the signal also depends on the poling direction axis, with electrical signal measured along the poling direction axis typically having a greater magnitude than the electrical signal measured along the other axes.
The poling direction of a piezoelectric material is selected during manufacture by subjecting a precursor material, e.g., a specific ceramic or ferroelectric material, to an initial large electrical field that causes the dipoles to become aligned. The material thereafter exhibits the piezoelectric effects described above when it is subject to smaller actuating fields or to mechanical forces.
Monolithic piezoelectric materials have been used in structural control as actuators or sensors. The materials can be bonded to or embedded in structural components, and then actuated by applying an electric field to deform the material and the structural component, or the material may be monitored to sense an electrical signal indicative of a mechanical force applied to the material and the structural component. For example, it has been suggested that piezoelectric materials could be incorporated in aircraft wings to detect and compensate for bending and vibration in the air foil caused by excessive lift and drag.
Many structural components, such as aircraft wings, are made of standard composite materials which are composed of relatively stiff reinforcing fibers, such as glass or carbon, embedded in a tough resin matrix. These composite materials must exhibit high strength, low weight, and resistance to environmental attack.